The present invention relates to compositions for providing phosphate conversion coatings on metal surfaces; processes for providing phosphate conversion coatings on metal surfaces, particularly copper; and the use of such compositions and processes in connection with sequences for fabricating printed circuits in which organic resists are utilized, and most particularly sequences in which electrophoretically deposited organic resins are employed as resists.
The provision of phosphate conversion coatings on metal surfaces is a well known surface treatment procedure. These coatings, which are formed in place on the surface of the metal substrate and incorporate metal ions dissolved from the surface, are known for use as undercoatings for paints applied, e.g., to zinc or iron or aluminum, as well as for use in forming, or aiding the formation of, corrosion-resistant surfaces. Typical commercial phosphating compositions comprise one or more metal phosphates (e.g., alkali metal phosphates) dissolved in phosphoric acid.
A recent advance in the fabrication of printed circuits also makes use of phosphate conversion coatings to passivate and uniformize copper surfaces prior to electrophoretic deposition thereon of organic resins to serve as resists (e.g., etch resists, plating resists) in printed circuit fabrication sequences. That advance proceeded from the recognition of fundamental problems encountered in the known use of electrophoretically depositable organic resins as resists on copper surfaces, such as localized insufficient protection of copper surfaces from etching when the resins are utilized as etch resists, and variable and inconsistent imaging and development properties when the resins are applied as photoresists. In turn, it was discovered that these problems were traceable to substantial non-uniformity of deposited resin layer thickness, density and consolidation. Still further, it was discovered that this non-uniformity was traceable to random, non-uniform oxidation of the exposed copper surfaces over which the resins are applied in printed circuit fabrication. At the time of electrophoretic deposition, then, the copper surfaces exhibit substantial non-uniformity in resistivity/conductivity characteristics; since the rate of electrophoretic deposition is proportional to the voltage difference between the applied voltage and that of the surface to which it is being deposited, point-to-point non-uniformity of the resistivity/conductivity properties of the copper surface can lead to substantial non-uniformity of thickness, density and consolidation of the resin layer applied thereon. In extreme cases, the deposited resin layer may be so thin in localized areas (sometimes to the point of being essentially non-existent, as in the form of a pinhole defect) as to afford insufficient protection to the underlying copper surface (e.g., against etching solutions), or may be so thick in localized areas as to make impossible the imaging and development needed to provide the desired resist patterns.
The recent advance in this regard involves the provision of the copper surfaces, prior to electrophoretic deposition of organic resin thereon, with a uniformizing/passivating coating, i.e., a coating which passivates the copper surfaces against random, non-uniform oxidation, and which substantially uniformizes the surfaces with respect to their resistivity/conductivity characteristics. In this manner, it becomes possible to apply to these surfaces a layer of electrophoretically deposited resin which, in its thickness, density and consolidation properties, is of the uniformity which is required of resists in printed circuit fabrication sequences. One such useful passivating/uniformizing coating for the copper surfaces is a phosphate conversion coating.
As further work has been undertaken with respect to this recent advance in printed circuit fabrication, it was determined that yet further improvement could be obtained if the phosphate conversion coating itself could be improved in its ability to passivate the underlying copper surfaces and in its ability to provide surfaces of uniform resistivity/conductivity characteristics for subsequent electrophoretic deposition of resin thereon to serve as a resist in printed circuit fabrication. Specifically, it was found that known phosphate conversion coating compositions and processes often provide only a minimal conversion coating on the copper surfaces. Although even the provision of only a minimal phosphate conversion coating is still quite useful in the fabrication of printed circuits using electrophoretically deposited resins as resists (i.e., providing a far more uniform and passivated surface for such deposition as compared to the case in which no phosphate conversion coating is used at all), overall process improvement and process control would be attainable if there could be provided phosphating compositions and processes which consistently produce a more substantial and uniform phosphate conversion coating on the copper surfaces.
With this specific need in mind, there have been developed novel phosphating compositions and processes having particular utility in providing passivating/uniformizing coatings on copper surfaces to which organic resins are to be electrophoretically applied so as to serve as resists in printed circuit fabrication processes. Yet further, the phosphating compositions and processes so developed have utility in all environments in which phosphate conversion coatings traditionally are employed.